I think you might be limited on that one. They do make downdraft units that fit behind slide-in ranges but I'm not too sure about cook tops.

??? They do make downdraft units that fit behind cooktops (the fan and motor still sit UNDER the cooktop) but I'm not too sure about slide-in ranges ;). I believe there is only one range that has an accompanying rear down draft, and only one stand-alone downdraft unit where the motor can be positioned on the back side of the popup unit to allow for an oven. But I don't remember the name of either offhand.

Plenty of manufacturers make in-counter downdraft vents (Thedrmadore, GE, Dacor, etc). Contact a local appliance dealer who can show you the options available to you.